This invention relates to methods of laminating composites in general, and more specifically to methods of laminating printed circuit boards, and to hydraulic presses used in these methods.
The demand for multilayer printed circuit boards has increased with the general trend toward miniaturization of electronic circuits. A multilayer printed circuit board incorporates multiple layers of circuitry separated by layers of dielectric material typically composed of woven glass impregnated with epoxy resin. The resin-impregnated glass firmly bonds together and electrically isolates the layers of circuitry. When properly laminated, a multilayer printed circuit board has uniform thickness, and each of the circuitry layers is precisely aligned with respect to the others. Such a multilayer board also has an even resin distribution with no air or volatiles trapped in the resin between the circuitry layers. These desirable qualities are even more important when the boards are used in harsh environments such as those typically found in military applications.
Multilayer boards are typically laminated by first interleaving layers of resin-impregnated glass with layers of circuitry, each layer of cicuitry being bonded to a respective side of a woven glass substrate for support. Several layers of resin-impregnated glass are usually placed in between the layers of circuitry. After the layers of circuitry are carefully aligned with respect to each other, a plurality of metal grommets are inserted through all the layers at their peripheries. The grommets serve to preserve the alignment of the layers of circuitry during the laminating process. The composite just described is then placed in a press where it is subject to pressure and heat, which cause the resin in the resinimpregnated glass to become substantially liquid. After a predetermined period of time, the temperature and pressure are both increased to solidify the resin and cure the composite into a printed circuit board.
The press typically used in the above-described process is a standard hydraulic press with a number of heated platens between which the composites are pressed. This type of press has a single piston rod for applying pressure at the centers of the platens and a plurality of platen guides, usually located at the corners of the platens, for guiding the platens during the pressing operation. A typical hydraulic press may have a number of additional platens disposed between the two outermost platens to form a plurality of openings, or "daylights," so that a number of composites may be simultaneously laminated within the press.
A number of problems have arisen from the use of this process in manufacturing multilayer printed circuit boards. It was discovered that the use of a single piston centrally located on the platens caused visible imperfections on the circuit boards, and that these imperfections could be prevented with the use of an increased amount of pressure. However, the use of greater pressure resulted in a number of other problems. Since the pressure was exerted at a single area on the platens, there was substantial variance in pressure across the platens, with the greatest pressure being exerted at the central portion of each platen and substantially less exerted about the periphery. When the resin in the composite became liquid during the heating process, the pressure gradient across the platens caused a substantial amount of resin flow from the central portion of each platen to the periphery. Substantial amounts of liquid resin would be squeezed out the sides of the composite, resulting in resin waste. As a result, more layers of glass were required between the layers of circuitry.
Even through an increased amount of pressure was used, underside air and volatiles still remained trapped within the resin between the layers of circuitry.
The pressure gradient also caused resin starvation of the middle of the multilayer boards, which resulted in lower dimensional stability. Multilayer boards produced in this manner were found to have substantial variations in thickness from their central portions to their peripheries, resulting in an undesirable increase in dielectric variance across the areas of the boards.
Applying pressure to a single, central area on the platens also caused increased slippage of the composite with respect to the platens, resulting in the need to use many grommets about the periphery of the composite in order to increase its coefficient of friction.
In order to increase the pressure uniformity, smaller composites were used which were substantially less in area than the area of the platens. The use of smaller composites greatly decreased the productivity of the presses used in the laminating process.
The center piston press used in this laminating process has a "restrictive frame," which typically has four bolster guide rods located at the corners of the platens to keep them horizontal. Each corner of the bolster plate has an aperture and a bushing within the aperture to accommodate the bolster guides. The bolster guides and the bushings are machined to extremely close tolerances in order to keep the bolster plate strictly parallel to the head plate. These rigidly parallel plates cause problems in the laminating process when a composite having nonuniform thickness is processed. Such a composite receives the most pressure at the thickest part and substantially less pressure at the thinner parts, causing a pressure gradient across the composite which contributes to the problem described above.
These hydraulic presses also have a system of hanger rods where more than two platens are displaced between the bolster and the head of the press. The hanger rods are used to support each of the intermediate platens so that a number of printed circuit boards can be simultaneously laminated. Each intermediate platen has its own set of hanger rods. As a result, many sets of hanger rods are required when many boards are to be simultaneously laminated. In addition, although some of the hanger rod systems are capable of being adjusted to vary the height of the intermediate platens, the adjustment mechanisms are crude, making it difficult to finely adjust the height of each platen within the press.